1. Field of the Invention
This invention relates to electrodes operating in a high temperature and erosive environment such as exists in an MHD generator channel.
2. Description of Material Art
A magnetohydrodynamic (MHD) generator transforms heat energy into electrical energy through utilization of the motion of an electrically conductive plasma in the presence of electric and magnetic fields. The motion induces an electromotive force between opposing electrodes, or walls comprising a plurality of electrodes which, in conjunction with insulating walls, form a duct. The plasma typically comprises ionized materials formed from combustion of fossil fuels such as coal, and includes a conductive seed material as well as combustion products. The plasma flows through the duct at a high temperature, on the order of 2500.degree. C., and at a high velocity, approximately 800 meters per second for systems operating in a subsonic regime, or approximately 1200 meters per second for systems operating in a supersonic regime.
The rapid motion of the high temperature plasma can seriously erode the electrodes. Additional damage can be caused from chemical or physical bonding of combustion products, typically referred to as slag, on the electrodes. Once bonding occurs, chemical reaction of the slag with the electrode can result in severe degradation. Damage can also be done to the electrodes by internal arcs, which are the primary mode of current transfer from the plasma to the electrodes. Damage from arcing is particularly severe when the electrode surface exposed to the plasma is below approximately 1800.degree. C. Even where the electrode surface becomes coated with slag, arcing can persist through the coating so as to damage the electrode surface. Several concepts have been proposed to alleviate electrode erosion and corrosion in MHD and other applications. For example, arc heaters have been tested where a magnetic field was applied in a manner to cause the arc to move along the electrode surface. However, the arcs still formed erosion tracks and had a tendency to stop moving and burn cavities in the electrode surface.
Attempts have also been made to continuously build up electrode surfaces in an MHD duct by continuously feeding material into the boundary layer formed between the electrode surface and the flowing plasma. While such systems appear promising, concerns are raised as to the excessive buildup and related heat transfer effects. Sacrificial electrodes have also been proposed, which, however, require a controlled motion of the electrode and associated control equipment and, would not eliminate slag deposition.
Also proposed have been so-called gas electrodes having a cavity or slots from which gaseous plumes are emitted into an MHD duct. The gaseous jet, which forms the electrode, is ionized in the cavity using microwave or radio frequency energy and, through use of a magnetic field, caused to swirl in the cavity in order to distribute current. It is difficult and power consumptive to achieve such ionization, and difficult to control.
It is therefore desirable to provide electrodes which can operate in high temperature erosive and corrosive environments such as that existing in MHD generator ducts.